(i) Field of the Invention
This invention relates to a small handpiece of the fluid driven turbine type useful for handcrafting or medical or dental treatment (hereinafter called "the small handpiece with a fluid driven turbine").
More specifically, this invention is concerned with a small handpiece with a fluid driven turbine, which has been dimensionally reduced by itself, has made it possible to increase within a head chamber the speed of turbine blades, that is, the output torque of a turbine rotor shaft having the turbine blades on a peripheral wall portion thereof and one of various tools in an axially-extending central portion thereof under supply of smaller energy, and shows high performance despite its small dimensions.
(ii) Description of the Related Art
Small handpieces with a fluid driven turbine have been used widely to date, for example, for cutting and/or grinding materials or, as medical or dental applications, for open-cutting, trepanating, cutting or severing bones or teeth upon conducting restorative operations in surgery (cerebral surgery, plastic surgery, laryngeal surgery or otolaryngology) or upon performing intraoral treatments in dentistry.
Among the above-mentioned small handpieces with a fluid driven turbine, small dental handpieces with a fluid driven turbine in which compressed air is used as the compressed fluid, i.e., a drive medium for the turbine, for example, are called "dental air turbine handpieces". They have an external appearance as shown in FIG. 1, which will be referred to upon describing the technical constitution of the present invention subsequently herein.
As is illustrated in FIG. 1, the dental air turbine handpiece A, described specifically, is composed of a head portion H and a grip portion G. A neck portion N of the grip portion G is connected continuously to the head portion H and is internally equipped with means for supply compressed air to air turbine arranged within the head H and discharging the compressed air from the head H. Designated at B in FIG. 1 is a tool fixedly held on a rotor shaft of the air turbine.
A description will hereinafter be made of the conventional art and the present invention by taking, for the convenience of description, the above-mentioned dental air turbine handpiece, which makes use of compressed air as a drive medium for the turbine, as a representative example of small handpieces with a fluid driven turbine.
Needless to say, the drive medium for the turbine is not limited to the above-mentioned compressed air in the present invention.
Namely, the compressed fluid as a drive medium for a turbine is not limited to compressed air but various compressed fluids, for example, compressed liquids and compressed gases including compressed steam, can be used.
The terms "air supply" and "air supply port" and the terms "air exhaust (discharge)" and "air exhaust port" as used in relation to compressed air in the subsequent description must therefore be read obviously as "supply" and "supply" port and "exhaust (discharge)" and "exhaust port" where other compressed fluids are applied.
Applications of small handpieces with a fluid driven turbine, which pertain to the present invention, are not limited to the above-mentioned dental field but are found in the medical field and also fields where materials are cut and/or ground.
In these application fields, the small handpieces according to the present invention can be used, needless to say, not only in a hand-held manner in association with the term "handpieces" but also as elements (members, parts or the like) of equipments.
A conventional dental air turbine handpiece A', especially, the internal construction of its head portion H' and its neck portion N' connected continuously to the head portion H' is illustrated in FIGS. 16 and 17.
In FIGS. 16 and 17 which illustrate the internal construction of the conventional dental air turbine handpiece A', FIG. 16 is a vertical cross-sectional view as viewed in the direction of an axis of a turbine rotor shaft 3' whereas FIG. 17 is a cross-sectional view taken in the direction of arrows XVII--XVII of FIG. 16.
As is depicted in FIG. 16, the head portion H' of the conventional dental air turbine handpiece A' has turbine blades 2' mounted on a peripheral wall of the turbine rotor shaft 3' and arranged in a chamber 11' of a head 1' and rotatably supports the turbine rotor 3' via bearings 4' disposed inside the head 1'.
The head 1' is composed of a head main body 12' and a cap portion 13'. Inside the head main body 12', the chamber 11' is formed to accommodate the turbine blades 2' and the bearings 4' are also arranged to rotatably support the turbine rotor shaft 3'.
Needless to say, a tool shaft 5' of a dental cutting or grinding tool or the like is fixedly held in an axially-extending central portion of the turbine rotor shaft 3' to perform various treatments. In addition, a chuck 51' for holding the tool shaft 5' as shown in the drawings is arranged on a peripheral wall of the tool shaft 5'. Although the illustrated chuck is a frictional check mechanism, a known touch-actuated chuck mechanism can also be used.
As is shown in FIG. 16, each bearing 4' is formed of a ball bearing which is in turn constructed of an inner race 41', an outer race 42', balls 43' and a retainer 44'. Along an outer peripheral wall or an end portion of each bearing 4', an O-ring or a known mechanism for enhancing the axial stiffness may be arranged to make the bearing 4' centripetal. Although the illustrated bearings are ball bearings making use of balls 43', they can be any known air bearing mechanism called an "air bearing".
A matter which requires special consideration in the structure of the conventional dental air turbine handpiece A' shown in FIG. 16 is the manner of arrangement of the supply and exhaust system for the compressed air and also the manner of arrangement of the turbine blades 2' in association with the supply and exhaust system within the chamber 11'.
The conventional art is characterized in that as shown in FIG. 16, the turbine blades 2' are arranged with an extremely large interval (d.sub.1 ') left between the turbine blades 2' and each of upper and lower, inner walls 111', 112' as viewed in the direction of the axis of the turbine rotor shaft 3'. In the drawing, the interval (d.sub.2 ') between a peripheral inner side wall 113' of the chamber 11' and the turbine blades 2' is generally narrower than the above-mentioned interval (d.sub.1 ').
In conventional products, for example, those having a turbine blade (2') height of 2.8 mm as viewed in the direction of the axis of the turbine rotor shaft 3', there are known products in which the interval (d.sub.1 ') is 1.150 mm (1150 .mu.m).
The above-described interval (d.sub.1 ') in the conventional products is extremely large compared with the corresponding interval in products according to the present invention. Its reason will be described subsequently herein when an air supply and exhaust system, the greatest characteristic feature of the present invention, is described.
As is illustrated in FIGS. 16 and 17, the neck portion N', specifically, its neck main body 6' is composed of one having:
an air supply channel 7' and an air supply port 71', both for supplying compressed air to the turbine blades 2' arranged within the chamber 11', and PA1 an air exhaust channel 8' and an air exhaust port 81', both for exhausting compressed air from the chamber 11'. PA1 an equation obtained by introducing "conditions for an isentropic flow" and "the equation of state of a gas" into an energy equation which had been obtained by integrating the Euler's equation of motion on a one-dimensional steady flow of compressible inviscid gas along a stream line, that is, into the Bernoulli's equation, and PA1 the Euler's equation of continuity for a one-dimensional steady flow, PA1 (1) When the pressure of compressed air was raised to increase the supply air velocity: PA1 (2) When the pressure of compressed air was raised to increase the air supply volume: PA1 (i) An increase in the air supply volume leads to an increase in the pressure within the chamber 11' and, as a result, a decrease is caused to occur in the supply air velocity. PA1 (ii) The compressed air so supplied collides against the turbine blades 2' and then circumferentially flows within the chamber 11' in the same direction as the direction of rotation of the turbine rotor shaft 3'. Compared with the speed of the turbine rotor shaft 3', the velocity of the circumferential flow is however extremely low so that the circumferential flow conversely begins to act as a resistance inside the chamber. This resistance becomes greater with the pressure of compressed air. PA1 (3) When the cross-sectional area of the air supply port was made greater to increase the air supply volume: PA1 They are totally different in the supply and discharge system for compressed air. Described specifically, the present invention has a single air supply port, whereas the U.S. patents have two air supply ports and the respective air supply ports are arranged at such positions as injecting compressed air against adjacent blades. PA1 Attention is now drawn to the interval (d.sub.1 ') between the turbine blades disposed within the chamber and each of the upper and lower, inner walls of the chamber, which has been described above with reference to FIG. 16. In view of FIGS. 3 and 9 of the U.S. patents, especially, the interval (d.sub.1 '), the handpiece disclosed in the U.S. patents is considered to employ an air supply and exhaust system which belongs to the conventional art. PA1 They are absolutely different from each other in the size of the compressed air supply port. In this respect, the U.S. patents do not disclose any specific quantitative values with respect to the sizes of the two air supply ports. In view of the embodiments of FIGS. 3 and 9, however, the U.S. patents disclose air supply ports having a size (diameter of each air intake port) equivalent to about 50% of the height of the turbine blades as viewed in the direction of the axis of the turbine rotor shaft. The total size of the two supply ports is considerably large. PA1 (i) said supply channel (7) has a single supply port (71); and PA1 (ii) a positional relationship between said single supply port (71) and an exhaust port (81) of said exhaust channel (8) is set so that said exhaust port (81) is arranged at a position proximal to said supply port (71).
It is to be noted that precisely speaking, the expression "for exhausting compressed air from the chamber 11'" is not correct, because the compressed air supplied from the air supply port 71' into the chamber 11' undergoes abrupt expansion and depressurization upon passing through the air supply port 71' and does not maintain the compressed state which the compressed air had at the time of its supply.
In the subsequent description, however, the above expression will be adopted in relation to the expression "compressed air supplied through the air supply port". Likewise, a flow of air in the chamber will also be described using the term "compressed air stream".
In the conventional dental air turbine handpiece A' described above, the head main body 12' of the head portion H' or the main body 6' of the neck portion N' may be provided with a lighting system for lighting a site under treatment or with a water supply channel for spraying water or physiological saline to a site under treatment so that cutting or grinding heat of a bone or tooth can be eliminated or the bone or tooth can be washed.
FIG. 17 illustrates the manner of supply and exhaust of compressed air in the conventional dental air turbine handpiece A'.
Described specifically, the supply and exhaust of compressed air in the conventional dental air turbine handpiece A' are conducted in the manner illustrated in the drawing, that is, by supplying the compressed air to the supply channel arranged in the main body 6' of the neck portion N', guiding the compressed air through the supply port 71' into the chamber 11', injecting the compressed air against the blades 2' to produce rotational drive force on the turbine rotor shaft 3' and then exhausting the compressed air from the chamber 11'.
Namely, the supply of compressed air against the turbine blades 2' and the exhaust of the compressed air from the chamber 11' in the conventional air turbine handpiece A' are conducted assuming as a prerequisite the existence of the air supply and exhaust system shown in FIG. 17.
The air supply and exhaust system will now be described in detail. As is depicted in FIG. 17, compressed air is injected against the turbine blades 2', which are arranged within the chamber 11', from the air supply channel 7' arranged in the main body 6' of the neck portion N' and then through the air supply port 71', the compressed air is caused to take a U-turn while flowing about the turbine rotor shaft 3' inside the chamber 11', and is then guided from the exhaust port 81', which is arranged in the main body 6' of the neck portion N', to the exhaust channel 8' for exhaustion. In FIG. 17, streams b of the compressed air within the chamber 11' are shown, in which solid lines indicate a circumferential flow through the interval d.sub.2 ' (see FIG. 16) and a dashed line designates a circumferential flow through the interval (d.sub.1) (see FIG. 16).
In the compressed air supply and exhaust system of the conventional dental air turbine handpiece A', compressed air is, as shown in FIG. 17, caused to take a U-turn (circumferential flows indicated by solid lines and the dashed line) while circumferentially flowing about the turbine rotor shaft 3' from the supply port 71' to the exhaust port 81' within the chamber 11' as indicated by the arrows b. This is believed to be attributable to the existence of the way of thinking as a fundamental that the flows of the compressed air continue to supply drive energy to the turbine blades in the course of their circumferential flows and hence to contribute to an increase in the rotational torque of the turbine rotor shaft 3'.
Since the compressed air supply and exhaust system of the conventional dental air turbine handpiece A' is based on the above-described way of thinking (design concept), the exhaust port 81' is arranged so that the compressed air injected through the supply port 71' is exhausted after the compressed air has flowed circumferentially in the chamber 11', specifically at the position shown in FIGS. 16 and 17.
Namely, as is shown in FIGS. 16 and 17, the exhaust port 81' is arranged at a position substantially symmetrical with the supply port 71' with a predetermined interval (C) left therebetween.
In view of the efficiency of transmission of moving energy from the compressed air to the turbine blades 2' and also the efficiency of exhaustion of the compressed air from the chamber, the supply port 71' is arranged so that as shown in FIG. 16, the compressed air is injected against substantially central parts of the turbine blades 12' as viewed in the direction of the axis of the turbine rotor shaft 3', whereas the exhaust port 81' is arranged at a position which is common in the central level to the supply port 71' and is spaced from the supply port 71' by the above-described interval (C). In view of the efficiency of exhaustion, an exhaust port generally has a greater opening area than an associated supply port.
To achieve an increase in the rotational torque (output) of the turbine rotor shaft 3' in the dental air turbine handpiece A' equipped with the air supply and exhaust system based on the conventional design concept described above, it is only necessary, theoretically speaking, to increase the supply velocity of the compressed air at the air supply port 71' or to increase the supply amount of the compressed air per unit time.
The rationale is that the force which the turbine rotor shaft 3' receives from the compressed air so supplied is equal to the moving energy which the turbine blades 2' receive from the streams of the compressed air per unit time, in other words, to the product of the supply velocity of the compressed air and the supply (inducted) volume of the air per unit time.
Further, the above-mentioned supply air velocity and supply (inducted) air volume are, as shown in FIGS. 18 and 19, dependent on the pressure of the compressed air so supplied and the cross-sectional area of the supply port. To achieve an increase in the torque (output), it is therefore only necessary to increase the pressure of the compressed air or to enlarge the cross-sectional area of the supply port. These measures have been adopted as routine approaches.
FIGS. 18 and 19 have been prepared as will be described next. In a system in which compressed air is supplied from a compressor and is injected into a chamber, an isentropic flow of compressible inviscid gas (i.e., a reversible flow not accompanied by any friction in an adiabatic system) was hypothetically considered. By using:
were determined flow velocities at an air supply port of the chamber (air supply velocities corresponding to predetermined compressed air pressures) (see FIG. 18) and mass flow rates (air supply volumes corresponding to predetermined compressed air pressures) (see FIG. 19). These flow velocities and mass flow rates were then plotted into graphs.
Phenomena which are to be described next are however actually observed when one attempts to increase the above-mentioned supply air velocity and supply air volume in a dental air turbine system accommodated in a chamber having air supply and discharge ports of predetermined sizes and a predetermined capacity.
Incidentally, the following observation results were obtained by conducting experiments while using an experimental apparatus fabricated with a transparent synthetic resin by copying, as an air turbine system, the conventional dental air turbine handpiece A' described above with reference to FIGS. 16 and 17, specifically, "JET MASTER FAR-E2" (trade name; manufactured by J. MORITA MFG. CORP.).
As is appreciated from FIG. 18, it was unable to increase the supply air velocity beyond the velocity of sound even if the pressure of compressed air was raised beyond 1 kgf/cm.sup.2. Compressed air pressures higher than the above level therefore do not contribute to an increase in torque.
The above-described proportionality began to break, leading to a deterioration in the efficiency of transmission of energy from the supplied air.
Described specifically, the compressed air so supplied was unable to increase the torque (maximum speed) of the turbine rotor 3' in proportion to an increase in the air supply volume, for example, in proportion to the increase in the air supply volume achieved by changing the pressure of compressed air from 2.0 kgf/cm.sup.2 to 3.0 kgf/cm.sup.2 shown in FIG. 19.
This can be attributed to the following reasons:
The compressed air so supplied began to act as a resistance in the chamber 11' as in the situation (2) described above. This tendency was however stronger than the above situation (2) that the pressure of compressed air to be supplied was increased.
This can be attributed to the fact that when the cross-sectional area of the air supply port increases, the compressed air injected through the air supply port is allowed to rapidly spread in the chamber 11' and its velocity is hence reduced to strengthen the resisting action. Accordingly, compressed air injected through a large air supply port encounters the above-mentioned resisting action, whereby the efficiency of transmission of energy from the supplied air to the turbine blade 2' is deteriorated further than the above-described situation (2).
Conventional techniques featuring enlargement of the air supply port as described above under (3) include, for example, the dental air turbine handpiece having dual air supply channel systems proposed in U.S. Pat. Nos. 3,893,242 and 4,020,556 to Lieb et al.
Each of the above U.S. patents provides the construction of the dental air turbine handpiece with new characteristic features in a wrench mechanism for fixing a took shaft on a turbine rotor shaft, an optical fiber system assuring efficient transmission of light (especially, connector means for optical fiber bundles in the interior of a handle portion, namely, a grip portion) and means for supplying compressed air to a turbine. FIGS. 2, 3 and 9 of the U.S. patents disclose an embodiment with the dual air supply channel systems (hence, having two air supply ports), in other words, an embodiment in which the cross-sectional area of the air supply port has been enlarged to increase the supply (inducted) volume of compressed air.
More specifically, the dental air turbine handpiece in each of the above U.S. patents has the structure that two air supply channels are arranged in the same horizontal plane relative to a turbine housing, in other words, two air supply ports are arranged at a desired angle relative to each other, compressed air is injected from each of the air supply ports against turbine blades disposed in the turbine housing and located in adjacent to the air supply port to apply rotating force to the turbine, and the air is then exhausted through exhaust ports.
In view of the description of the specification of each of the U.S. patents and FIGS. 3 and 9 in the patent, the exhaust ports are arranged above and below the air supply port, respectively.
A description is now made of significant differences in construction between the dental air turbine handpiece disclosed in the above U.S. patents and that of the present invention. These differences lead substantial differences in advantageous effects therebetween. This matter will be described in detail subsequently herein on the basis of substantiating data.
Compared with the dental air turbine handpiece according to the present invention, that disclosed in the U.S. patents is different in the following points:
The handpiece according to the present invention, on the other hand, is provided with only one air supply port as described above. Further, the size of the single air supply port is as small as about 50% of the height of the turbine blades.
As is evident from the foregoing, the dental air turbine handpiece of the U.S. patents was constructed in a way of thinking totally different from the below-described design concept of the present invention, and is believed to have adopted the approach that the number of air supply ports is increased to enlarge the overall cross-sectional area of air supply ports, in other words, to have adopted the approach that the cross-sectional area of an air supply port is increased to make the air supply volume greater for an increase in the torque of the turbine rotor shaft.
The above U.S. patents, however, exerted ingenuity in the manner (positions) of arrangement of the two air supply ports. Compared with simply enlarging the cross-sectional area of a single air supply port as explained above under (3), the compressed air so supplied is allowed to spread at a lower rate in the chamber 11'. The efficiency of transmission of energy from the compressed air so supplied is hence improved correspondingly, but is still poor.
In the conventional dental air turbine system, it may also be contemplated for the elimination of the above-described drawback to make the air exhaust port greater relative to the air supply port so that the resisting action of the compressed air can be eliminated.
If the air exhaust port is made larger relative to the air supply port, however, the compressed air injected from the air supply port is allowed to rapidly spread in the chamber and is then exhausted. The amount of compressed air which collides against the turbine blades 2' is therefore decreased, so that the efficiency of transmission of the energy of the compressed air so supplied is deteriorated, resulting in a sharp decrease in the torque (output) of the turbine rotor 3'.
Limitations, which the conventional are described above with reference to FIGS. 16 and 17 and various improvements including those proposed in the above U.S. patents are accompanied with, will be described subsequently herein on the basis of substantiating data upon description of the technical features of the present invention.